Many devices include permanent magnets. For example, some electric machines, such as electric motors and electric generators, include a permanent-magnet type rotor that includes permanent magnets. Many permanent-magnet type rotors have a rotor body with permanent magnets attached to an outer surface of the rotor body by adhesive bonds. Such adhesive bonds have limited strength and may fail and allow the permanent magnets to detach from the rotor body under some circumstances, such as during high-speed rotation of the rotor.
U.S. Pat. No. 6,087,751 to Sakai (“the '751 patent”) shows a reluctance type rotating electric machine having a rotor that includes permanent magnets mounted inside cavities in the rotor. The rotor of the '751 patent includes a rotor core constructed of laminated steel plates. The rotor core includes a plurality of cavities. The permanent magnets are contained in the cavities of the rotor core.
Although the rotor of the '751 patent includes permanent magnets mounted in cavities of the rotor core, certain disadvantages persist. For example, each of the cavities has a perimeter that is composed of substantially straight portions and sharp corner portions between the substantially straight portions. The sharp corner portions of the perimeters of these cavities may create large stress concentrations in the rotor core. Such large stress concentrations in the rotor core may adversely affect the durability of the rotor core.
The cavities, permanent-magnet pieces, rotor, and electric machine of the present disclosure solve one or more of the problems set forth above.